Electrostatic pinning is one method used in the casting of an extruded plastic resin to a surface. A pinning wire is raised to a high voltage such that it ionizes the air around it, creating a corona discharge. Ions of the same polarity as the wire are repelled towards the resin and, hence, electrostatically charge the resin. The electrostatic charge, in combination with the ground plane provided by the casting surface, exerts an electrostatic force on the resin that improves its contact with the casting surface, resulting in films that are rapidly quenched and free of defects. This is especially important at increased casting speeds where air entrainment between the resin and the casting surface would otherwise result in film nonuniformities and defects.
The use of an electrostatic pinning wire is described in U.S. Pat. No. 3,223,757. In the process described in this patent, a film forming polymeric material is extruded onto an electrically grounded quenching surface while being passed in close proximity to an electrode. The electrode deposits an electrostatic charge onto the polymeric material before the material has solidified which causes the material to adhere firmly across the width to the quenching surface.
In U.S. Pat. Nos. 3,655,307; 3,660,549; and 3,686,374, a method is disclosed for improving the high speed performance of electrostatic pinning by using a grounded shield in the shape of a semi-cylinder that is covered with an insulating layer. The second electrode gets charged from the pinning wire tending to push the ions towards the resin. In addition, a gas may be introduced that serves to raise the sparkover voltage, the voltage at which the electrode discharges onto the wheel or the die.
In U.S. Pat. No. 3,820,929, a second wire is electrically connected to the pinning wire. It is of a larger diameter so as not to emit ions, hence, it serves in a similar capacity as the dielectric shield mentioned in the above paragraph except the geometry is more limited.
In U.S. Pat. No. 4,129,630 a non-grounded shield, conductive or non-conductive and of variable geometry, is interposed between the pinning wire and the extrusion die, and claimed to improve pinning due to improved electric field uniformity resulting from decreased contamination of the pinning wire with sublimate from the die.
A semi-cylindrical, grounded conductive shield is disclosed in U.S. Pat. No. 4,244,894. This patent asserts that at a given pinning wire voltage, more charge is delivered to the resin with a grounded shield than with other configurations.
In U.S. Pat. No. 4,534,918, a second electrode consisting of grounded pins extended towards the pinning wire is disclosed. This patent claims that these pins improve the uniformity of the charge laid out on the resin resulting in better pinning.
In U.S. Pat. No. 5,030,393, a set of floating conductive electrodes are disposed to either side of the pinning wire. These electrodes perform similarly to the insulating shields mentioned earlier, such as in U.S. Pat. No. 3,655,307.
Finally, another method that is used is vacuum pinning. However, it is believed that vacuum pinning is very dependent on the usage of beads on either side of the extruded resin. For applications requiring minimal beads, beads on only one surface or no beads at all, vacuum pinning is difficult and may not work at all.
It has been observed that pinning at higher speeds requires higher current output by the pinning wire. This can be increased by raising the wire voltage, however, above a certain voltage arcing will occur between the wire and either the casting surface or the die. The above described methods have been employed to circumvent this limitation. However, none have been completely successful.
The present invention is a method that solves the above-described problems by using an electrically biased conductive shield in conjunction with a pinning wire so that the charging efficiency of the pinning apparatus can be controlled, resulting in improved pinning latitude with respect to polymer formulation and process speed.